Personal Rapid Transit (PRT)
Passengers Per Hour (PPH) Capacity
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System Capacity or Volume

Can small cabs move large numbers of people like traditional mass transit? Yes. Uninterrupted flow is the key to capacity, not vehicle size. For example, 60-passenger buses arriving two minutes apart (a very high flow rate for an American bus system) can carry 1800 passengers per hour. PRT vehicles coming every two seconds can provide the same capacity.  PRT capactiy depends on headways:

  • 0.5 second = 120/min or 72000hr
  • 0.6 second = 100/min or 6000/hr
  • 2.0 seconds = 30/min or 1800/hr

A commonly accepted safety zone on roadways is 2 seconds between cars. Although automatic control of PRT cabs is safer and more reliable than human drivers, let's assume our PRT systems starts with that comfortable two seconds of space between each cab, aka "headway". At that headway, 1800 cabs per hour can roll down the guideway. That's 1800 people per hour assuming sole-ridership will prevail (30 cabs/min * 60 mins/hour = 1800 cabs per hour). That approximates the maximum volume of a freeway lane of traffic (2200). After a few years of operation, we may have the confidence to reduce the headway times to only one half second. That would quadruple throughput to 7200 cabs/hour. Now we're talking the volume of three freeway lanes in less than the space of one physical lane.

Now, compare that volume to LRT and trains. Although LRT systems may be designed for high volume, the actual limit of any operating LRT system in the U.S. is 1200 riders per hour; peak  in Sacramento is about 1000 passengers/hr.  Likewise for trains where the theoretical limit is 20,000 riders/hour, actual loading often tops out near 7000 riders/hour. An exception may be BART where reports indicate near-saturation of the trans-Bay tube at 20,000 riders/hour [is that one way, or both?].

Another capacity comparison could be made with computer controlled cars as demonstrated near San Bernadino, CA.  Partners for Advanced Transit and Highways (PATH) ran Buick Le Sabres by computers on a dedicated strip of freeway with magnets embedded so the cars could be computer controlled. They ran for thousands of miles at 60 mph with 0.25 sec. headways.  Some of PATH's research, particularly its work in the Advanced Vehicle Control Systems area, has been covered by a range of media. More recently, Hundai has demonstrated even more control of vehicles at speed.

Speed is another factor in capacity. Here are critical ideas from PRT pioneer Ed Anderson:

Subj: RE: [prt-talk] Digest Number 56
Date: 5/27/01 5:32:19 PM Pacific Daylight Time
From: (Ed Anderson)

You mentioned some of the system problems. Tires vs. maglev are not the most important considerations. Curve radii increase as the square of the speed and off-line guideway lengths increase in proportion to speed. These are the most important factors. Life-cycle-cost per passenger-mile is the annualized capital + operating cost divided by the annual ridership. Costs increase with speed regardless of the means of suspension and ridership will increase with speed to a point. After a certain speed, costs increase faster than ridership so the cost per passenger-mile increases. - JEA

So, pick a speed that ensures high ridership by offering 1) a low cost per passenger-mile and 2) speeds that compete with the automobile . Absent any analysis, I pick 40 mph. Let's start engineering with that operating speed in mind.

Here's some capacity numbers from the bike folks: It takes three lanes of a given size to move 40,000 people across a bridge in one hour using automated trains, four to move them on buses, twelve to move them in their cars, and only two lanes for them to pedal across on bicycles.

A vehicle at a red light requires about 240 square feet of space (that's a standard 12-foot lane with a standard 20-foot long "envelope" per car). At 20 mph, it requires about 700 square feet.  And for a car zooming at 40 mph, the number balloons to about 2,000 square feet.  Maximal traffic on a highway lane runs at 2.2-second intervals.  At 10 mph that is 1000/hr; at 30 mph about 1500/hr.  It never gets more about 1500/hr because the vehicle grows with velocity.  At 50 mph,  a car is 1,285 feet long.  PRT capacity or speed does not decrease with a heavy load; at 2 second headway, it will have 3 times the capacity as a landeof traffic, and at 0.5 second headway, it will have 12 times the capacity.  

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